The invention relates to a material measure for a distance measuring system.
Such material measures are known from German Patent Disclosure DE 199 22 363, which is hereby incorporated in full as a reference. It shows a material measure which has a base body 3, to which a first tape measure 29 with perforations 37 is secured.
The base body 3 is formed by the guide rail of a linear anti-friction guideway. The center line, not shown, of the tape measure is therefore a straight line. On the guide carriage 19 of this linear anti-friction guideway, there is a sensor head 31, with which the tape measure can be scanned inductively to ascertain distance information. As perforations, rectangles are provided, which extend transversely to the center line of the tape measure and are located periodically. The tape measure is of steel and has a thickness of approximately 0.3 mm. The width of the rectangle and the spacing in the longitudinal direction is 0.5 mm each, so that the inductive sensor head outputs sine and cosine signals with a period of 1 mm. The period of the tape measure is set exactly by means of a suitable tensing or stretching thereof. The stretched tape measure is secured to the base body by means of spot welds, and it is received in an indentation which is closed with a covering tape 15.
The perforations in the tape measure are made by means of etching. To that end, a flat, unperforated steel band is coated with photoresist and exposed to light with high precision using the desired perforation pattern. The photoresist is then developed and at the places where the perforations are provided it is removed, while it still covers the steel band at the other places. This process is performed on both sides of the tape measure. By dipping the pretreated band into an etching fluid, the perforations are chemically dissolved out of the band. The problem here is that the boundary faces of the perforations, which extend in the thickness direction of the tape measure, have an irregular shape. These irregularities depend on both the process conditions in etching, such as the etching time, and on the geometry of the tape measure as well as its period and thickness. In inductive scanning of the tape measure, these irregularities lessen the precision of the entire distance measuring system.
This leads to conflicting goals, since the thickness of the tape measure should be as great as possible on the one hand, so that as strong a sensor signal as possible can be generated, and as small as possible on the other, so that the least possible irregularity of the perforations will be obtained. Similarly conflicting goals exist in terms of the period of the tape measure. If the period is short, the sensor can generate a more-precise measurement signal, while the irregularity of the perforations of the tape measure increases.